1. Field of the Invention
The present invention relates to chip stacks in which a plurality of integrated circuit chip packages arranged in a stack are electrically connected in a desired fashion and to methods of making such stacks.
2. History of the Prior Art
Various common approaches are used to increase memory capacity on a circuit board. Larger memory IC chips can be used, if available. The size of the circuit board can be increased in order to hold more IC chips. Vertical plug-in boards can be used to increase the height of the mother board. The memory devices can be stacked in pancake style (sometimes referred to as 3D packaging or Z-Stacking). The Z-Stacking approach interconnects from 2 to as many as 8 chips in a single component which can be mounted on the "footprint" of a single package device. This approach is the most volumetrically efficient. Package chips in TSOP (thin small outline package) or LCC (leadless chip carrier) form have been used for stacking, and are perhaps the easiest to use. Bare chips or dies have also been used, but the process for forming a stack thereof tends to be complex and not well adapted to automation.
In forming a stack of IC chips such as memory chips, the chips must be formed into a stack and at the same time must be electrically interconnected in desired fashion. Typically, the chips, which are mounted within packages therefor, have most of the electrical contacts thereof coupled in common or in parallel to contacts on a supporting substrate, and several unique contacts which are coupled individually to the substrate to the exclusion of the other chips. The prior art includes various different arrangements for electrically interconnecting the IC chips in a stack. For example, electrical conductors which may comprise thin film metal on an insulating base may be disposed perpendicular to the planes of the planar chips so as to connect those conductors on each chip which are exposed through openings in an insulating layer. Where the chip packages are assembled into a stack, electrical connections may be accomplished by lead frames or solder strips extending along the sides of the stack and attached to the electrical contacts of the chips.
Another common technique for providing the desired electrical interconnections in a chip stack is to form a stack of chips having bonding pads disposed on the chips adjacent the outer edges thereof. After assembling the stack of chips, the chip edges are ground flat and polished before sputtering an insulating layer thereon. The bonding pads on the edges of the chips are masked during the sputtering process to avoid covering them with the insulating layer. Next, a metal layer is sputtered onto the entire edge of the stack in conjunction with photomasking which forms conductor traces of the metal layer in desired locations for connecting the bonding pads.
Further examples of vertical stacks of IC chips and various methods of making such stacks are provided by U.S. Pat. Nos. 4,956,694, 5,313,096 and 5,612,570, which patents are commonly assigned with the present application. U.S. Pat. No. 5,612,570, which issued Mar. 18, 1997 and is entitled CHIP STACK AND METHOD OF MAKING SAME, describes a chip stack and a method for making the same in which chip packages are first assembled by mounting plastic packaged chips or thin, small outline package chips (TSOPs) within the central apertures of thin, planar frames having a thickness similar to the thickness of the packaged chip. Leads at opposite ends of the package are soldered to conductive pads on the upper surface of the surrounding frame. Each frame also has other conductive pads on the upper and lower surface thereof adjacent the outer edges of the frame, which are coupled to the conductive pads that receive the leads of the packaged chip by conductive traces and vias. A chip stack is then formed by stacking together a plurality of the chip packages and dipping the outer edges of the stack into molten solder to solder together the conductive pads adjacent the outer edges of the frames. The conductive pads adjacent the outer edges of the frame can be interconnected in a stair step arrangement, and pads on opposite sides of each frame can be coupled in offset fashion using vias, in order to achieve desired electrical interconnections of the various chips.
A still further example is provided by copending application Ser. No. 08/935,216, filed Sep. 22, 1997 and entitled CHIP STACK AND METHOD OF MAKING SAME. Such application, which is commonly assigned with the present application, describes formation of a stack of ball grid array chip packages by assembling a ribbon-like structure of thin, planar bases, each with plural terminals and an interconnecting conductive pattern thereon, and with the bases electrically interconnected by flex circuits extending therebetween. A different chip package is mounted on each base by soldering the balls of a ball grid array thereon to the terminals of the base. The bases alternate in orientation, so that alternate chip packages are joined to the tops and bottoms of the bases. The resulting arrangement is then folded over on itself, with chip packages being joined to adjacent bases using adhesive. The resulting chip stack is mounted on a substrate by soldering the balls of the ball grid array at the underside of the lowermost base at the bottom of the stack to the substrate. The conductive patterns on the bases and the interconnecting flex circuits form conductive pads which contact selected terminals of the various chip packages as the paths extend in alternating fashion through the chip stack between opposite sides of the stack.
A still further example is provided by copending application Ser. No. 08/971,499, filed Nov. 17, 1997 and entitled METHOD OF MAKING CHIP STACKS. Such application, which is commonly assigned with the present application, describes the making of a chip stack which begins with the formation of a plurality of panels having apertures therein and conductive pads on opposite sides thereof. Solder paste is deposited on the conductive pads prior to mounting plastic packaged IC chips within each of the apertures in each of the panels so that opposite leads thereof reside on the conductive pads as opposite sides of the apertures. The plural panels are then assembled into a stack, such as by use of a tooling jig which aligns the various panels and holds them together in compressed fashion. The assembled panel stack is heated so that the solder paste solders the leads of the packaged chips to the conductive pads and interfacing conductive pads of adjacent panels together, to form a panel stack comprised of a plurality of chip package stacks. Following cleaning of the panel stack to remove solder flux residue, the individual chip package stacks are separated from the panel stack by cutting and breaking the stack. Score lines across the topmost panel and transverse slots within remaining panels therebelow result in the formation of strips of chip package stacks when longitudinal cuts are made through the panel stack. The remaining portions of the uppermost panel within such strips are then snapped along the score lines thereof to separate the individual chip package stacks from the strips.
The various arrangements and methods described in the patents and patent applications noted above have been found to provide chip stacks and methods which are advantageous and which are suited for many applications. Nevertheless, the provision of further alternative arrangements and methods would be advantageous. In particular, it would be advantageous to provide chip stacks and methods of making such stacks which utilize available materials and known process techniques. The assembly of such stacks should lend itself to automated production methods, and thus would be competitive with other stacking approaches.